The present invention relates to coatings applied to metal components of gas turbine engines, radial inflow compressors and radial turbines, including micro-turbines and turbo-chargers, that are exposed to high temperature environments and, in particular, to a new type of abradable coating applied to turbine shrouds used in gas turbine engines in order to improve the performance and efficiency of the turbine blades (also known as xe2x80x9cbucketsxe2x80x9d). Although the present invention has been found particularly useful in stage 1 turbine shrouds, the same coating developments can be used in other stages of gas turbine engines, as well as on hot gas path metal components of other rotating equipment exposed to high temperature environments. The present invention can also be used to repair and/or replace the coatings on metal components already in service, such as coated turbine shrouds.
Gas turbine engines are used in a wide variety of different applications, most notably electrical power generation. Such engines typically include a turbocompressor that compresses air to a high pressure by means of a multi-stage axial flow compressor. The compressed air passes through a combustor which accepts air and fuel from a fuel supply and provides continuous combustion, thus raising the temperature and pressure of the working gases to a high level. The combustor delivers the high temperature gases to the turbine, which in turn extracts work from the high pressure gas working fluid as it expands from the high pressure developed by the compressor down to atmospheric pressure.
As the gases leave the combustor, the temperature can easily exceed the acceptable temperature limitations for the materials of construction in the nozzles and buckets in the turbine. Although the hot gases cool as they expand, the temperature of the exhaust gases normally remains well above ambient. Thus, extensive cooling of the early stages of the turbine is essential to ensure that the components have adequate life. The high temperature in early stages of the turbine creates a variety of problems relating to the integrity, metallurgy and life expectancy of components coming in contact with the hot gas, such as the rotating buckets and turbine shroud. Although high combustion temperatures normally are desirable for a more efficient engine, the high gas temperatures may require that air be taken away from the compressor to cool the turbine parts, which tends to reduce overall engine efficiency. One aim of the present invention is to enable the stationary shroud to cope with the high gas temperatures without having to increase cooling air.
In order to achieve maximum engine efficiency (and corresponding maximum electrical power generation), it is also important that the buckets rotate within the turbine housing or xe2x80x9cshroudxe2x80x9d without interference and with the highest possible efficiency relative to the amount of energy available from the expanding working fluid.
During operation, the turbine housing (shroud) and a portion of the hub remain fixed relative to the rotating buckets. Typically, the highest efficiencies can be achieved by maintaining a minimum threshold clearance between the shroud and the bucket tips to thereby prevent unwanted xe2x80x9cleakagexe2x80x9d of gas over or around the tip of the buckets. Increased clearances will lead to leakage problem can cause significant decreases in overall efficiency of the gas turbine engine. Only a minimum amount of xe2x80x9cleakagexe2x80x9d of the hot gases at the outer periphery of the buckets, i.e., the small annular space between the bucket tips and turbine housing, can be tolerated without sacrificing engine efficiency.
The need to maintain adequate clearance without significant loss of efficiency is made more difficult by the fact that as the turbine rotates, centrifugal forces acting on the turbine components can cause the buckets to expand radially in the direction of the shroud, particularly when influenced by the high operating temperatures. Thus, it is important to establish the lowest effective running clearances between the shroud and bucket tips at the maximum anticipated operating temperatures.
A significant loss of gas turbine efficiency can also result from wear of the bucket tips if, for example, the shroud is distorted or the bucket tips rub against the shroud creating metal-to-metal contact. Again, any such deterioration of the buckets at the interface with the shroud when the turbine rotates will eventually cause significant reductions in overall engine performance and efficiency.
In the past, abradable type coatings have been applied to the turbine shroud to help establish a minimum, i.e., optimum, running clearance between the shroud and bucket tips under steady-state temperature conditions. In particular, coatings have been applied to the surface of the shroud opposite the buckets using a material that can be readily abraded by the tips of the buckets as they turn inside the housing at high speed with little or no damage to the bucket tips. Initially, a small clearance exists between the bucket tips and the coating when the gas turbine is stopped and the components are at ambient temperature. Later, during normal operation, the centrifugal forces and increased heat generated by the system inevitably results in at least some radial extension of the bucket tips, causing them to contact the coating on the shroud and wear away a part of the coating to establish the minimum running clearance. As detailed below, the relationship between the type of material used to form the abradable coating and the temperature of the turbine shroud can play a critical role in the overall efficiency and reliability of the entire engine. Without abradable coatings, the cold clearances between the bucket tips and shroud must be large enough to prevent contact between the rotating bucket tips and the shroud during later high temperature operation. With abradable coatings, on the other hand, the cold clearances can be reduced with the assurance that if contact occurs, the sacrificial part will be the abradable coating and not the bucket tip.
As noted in prior art patents describing abradable coatings for use in turbocompressors and gas turbines (see e.g., U.S. Pat. No. 5,472,315), a number of design factors must be considered in selecting an appropriate material for use as an abradable coating on the shroud, depending upon the coating composition, the specific end use, and the operating conditions of the turbine, particularly the highest anticipated working fluid temperature. Ideally, the cutting mechanism (e.g., the bucket blade tips) can be made sufficiently strong and the coating on the shroud will be brittle enough at high temperatures to be abraded without causing damage to the bucket tips themselves. That is, at the maximum operating temperature, the shroud coating should be preferentially abraded in lieu of any loss of metal on the bucket tips.
Thus, the need exists for an abradable coating system that will allow for the use of bucket tips at elevated temperatures without requiring any tip reinforcement (such as the application of aluminum oxide and/or abrasive grits such as cubic boron nitride). A need also exists for an improved abradable coating system that can be used if necessary in conjunction with reinforced bucket tips in order to provide even longer term reliability and improved operating efficiency.
In addition, any coating material that is removed (abraded) from the shroud should not affect downstream engine components. The abradable material must also be securely bonded to the turbine shroud and remain bonded while portions of the coating are removed by the bucket blades during startup, shut-down or a hot-restart. Preferably, the abradable coating material remains bonded to the shroud for the entire operational life of the gas turbine and does not significantly degrade over time. Ideally, the coating should also remain secured to the shroud during a large number of operational cycles, that is, despite repeated thermal cycling of the gas turbine engine during startup and shutdown, or periodic off-loading of power.
Another critical design factor that must be considered in the context of abradable shroud coatings concerns the rate of degradation of the coating due to exposure to hot gases containing oxygen over long periods of time at elevated temperatures. Many prior art coatings require bucket tip reinforcement, particularly in higher temperature applications. As the gas temperature increases, coating structures become more and more ductile and the increased ductility tends to reduce the ability of the coating to be abraded. Thus, most of the prior art coatings use higher levels of porosity to compensate for the increased ductility. However, the higher porosity also tends to reduce the life span of the prior art coatings at high temperatures because the same porosity volume that make the coatings less ductile also renders them much more vulnerable to oxidation, particularly in the earlier turbine stage conditions.
In the past, a number of abradable coatings have been suggested for use on compressor shrouds and other gas turbine components. The coatings in U.S. Pat. Nos. 3,346,175; 3,574,455; 3,843,278; 4,460,185 and 4,666,371 represent a few well known abradable coatings that have been used with some success on metal shrouds. However, these conventional coatings are not sufficiently durable or resistant to oxidation in much higher temperature environments. Thus, the prior art coatings tend to oxidize, delaminate or even separate from the shroud substrate as the turbine undergoes thermal cycling during startup and shut down.
Over the past twenty years, considerable research and development work has been done (including by General Electric) in the field of high temperature coatings to solve these known abradability and oxygen-resistance problems. The result has been an increase in the capability of the coatings to resist degradation over long periods of time.
The problems of abradability and oxygen resistance for turbine shrouds remain, however, and have become more pronounced in recent times because of the desire to use even higher operating temperatures in gas turbine engines to thereby increase their working efficiency. As the operating temperatures go up, the durability of the engine components must correspondingly increase. One known shroud coating available commercially utilizes a metallic layer formed from an oxidation-resistant alloy known as xe2x80x9cMCrAlYxe2x80x9d in combination with a polymer material, such as polyester or polyimide (used to impart porosity), where xe2x80x9cMxe2x80x9d can be iron, cobalt and/or nickel.
Another recognized improvement in shroud coatings for mid- to high temperature applications uses a thermal barrier coating in addition to an abradable top coating. Such thermal barriers can be formed of various non-porous materials including alloys and ceramics such as zirconia stabilized by an oxide material or MCrAlY, where xe2x80x9cMxe2x80x9d consists of iron, cobalt or nickel.
The present invention concerns a high temperature abradable coating system for turbine shrouds that is much more effective than conventional prior art systems, both as an abradable coating and as an oxidation-resistant component, particularly at operating temperatures above 1400xc2x0 F. The coatings in accordance with the invention also provide close clearance control between the bucket tips and shroud, and thereby reduce hot gas leakage and improve overall gas turbine efficiency.
The coatings in accordance with the invention are much more effective in controlling oxidation than the current state of the art coatings, such as Sulzer Metco SM2043 which consists of MCrAlY together with 15 wt % polyester and 4 wt % boron nitride (hBN). See U.S. Pat. No. 5,434,210. The MCrAlY component of the SM2043 nominally contains CO25Ni16Cr6.5Al0.5Y and is recommended for applications up to approximately 1380xc2x0 F. without tipped (uncoated) buckets and 1560xc2x0 F. for tipped buckets. Because the SM2043 material does not abrade well above 1380xc2x0 F., it can result in non-uniform wear of the shroud coating and/or cause damage to the bucket tips themselves by the rotational impact of the bucket with the shroud metal, ultimately requiring some type of tip reinforcement or coating.
In addition, because of the high porosity in coatings using Sulzer Metco SM2043, the oxidation life of such coatings is relatively short at operating temperatures above 1580xc2x0 F. For example, the SM2043 coatings begin to show poor oxidation resistance at temperatures above 1380xc2x0 F. and the resistance level deteriorates significantly above that temperature, with many coatings lasting only a few hours at temperatures approaching the level of earlier turbine stages (1700xc2x0 F.). The poor oxidation resistance of these prior art compositions is attributable to the relatively high porosity levels (about 55% by volume) in the abradable top coat and to the poor oxidation resistance of CoNiCrAlY in such high temperatures. The high coating porosity tends to allow a much higher rate of ingress of oxygen into the coating.
Thus, a significant need exists in the art for an abradable coating for gas turbine shrouds operating at higher than average temperatures, i.e., above 1380xc2x0 F., which is capable of achieving a longer oxidation life, preferably up to 24,000 hours, when used at gas temperatures in the 1600-1850xc2x0 F. range. There is also a significant need for improved abradable coatings capable of ensuring that the turbine buckets suffer from only minimal wear during startup and shutdown due to radial expansion and contraction. There is also a need to provide an abradable coating that will avoid the necessity for tipped blades which might otherwise be required due to the non-abradable nature of coatings in the higher temperature ranges of turbine shrouds. Finally, a need exists to provide a coating that will have sufficient erosion resistance over the life of the gas turbine equipment, thereby avoiding the need to interrupt operation to maintain and/or replace the turbine coating.
It has now been found that the above requirements for an improved abradable metallic coating system in turbine shrouds can be satisfied by using a coating containing the following basic components:
1. A xe2x80x9cfugitivexe2x80x9d polymer or other plastic phase (such as polyester or polyimide) which can then be burned off without leaving any residue or ash to create a porous coating. The porosity level can then be optimized for maximum abradability and oxidation life. As detailed below, a coating having about 12 wt % polyester has been found to exhibit excellent abradability for applications involving turbine shroud coatings. It has also been found that abradable coating thickness in the range of between 40 and 60 mils will provide the best performance for turbine shrouds exposed to gas temperatures between 1380xc2x0 F. and 1850xc2x0 F.
2. A metallic oxidation-resistant matrix phase such as CoNiCrAlY, e.g., Praxair Co211 (Co32Ni21Cr8Al0.5Y), NiCoCrAlY, FeCrAlY or NiCrAlY, e.g., Praxair Ni211 (Ni22Cr10Al1Y); and
3. A brittle intermetallic phase, such as xcex2-NiAl (68.51 wt % Ni and 31.49 wt % Al), or an intermetallic phase former that serves to increase the brittle nature of the metal matrix and thereby increase the abradability of the coating at elevated temperatures. The use of this third phase also significantly improves oxidation resistance at high temperature without adversely affecting abradability.
Abradable coatings using components (1) and (3) above have been found particularly useful for E-Class, land-based shrouds and other applications where the buckets are not normally tipped (coated) and the shroud is exposed to high operating temperatures at or near 1700xc2x0 F.
Coatings in accordance with the above three basic components can be applied to both new and used turbine shrouds in gas turbine engines using conventional techniques (such as plasma spray), or to other hot gas path metal components of rotating equipment exposed to high temperatures. For example, the coatings on existing gas turbine engine shrouds can be physically removed after the equipment is taken out of service for repair or routine maintenance, with the new coatings then being applied using conventional high level bonding and coating techniques known to those skilled in the art.